Supernova déjà vu, all over again

When astronomers saw a star
explode they knew – thanks to
Einstein – that they could watch
it again a year later. Katie Mack explains.

In late 2015, the Hubble Space Telescope
turned toward a distant galaxy to watch
the explosive demise of a doomed star.
Supernova Refsdal was a replay, having
already been seen and measured a year
before. But between here and there was a
region of space so crowded and mangled
with galaxies, the exploding star’s light
rays flowed through it like a river over
rapids, twisting and bending along
multiple paths. Hubble was watching
another angle on the same explosion, from
light rays that took the scenic route.

That light can split and bend is a
familiar concept. Every time we look
through a lens, see a reflection off glass, or
watch the dance of sunlight underwater,
we are watching light being bent or split by
the matter through which it flows.
In space, as long as it doesn’t run into
anything, such as a galaxy, there’s no
matter for light to flow through: it should
travel in straight lines, unimpeded.

But
thanks to Einstein we know it doesn’t. He
hypothesised that when there’s matter
nearby, empty space itself can bend,
stretch and compress, carrying light beams
along with it. In fact, this phenomenon of
gravitational lensing was one
of the first pieces of evidence to support
Einstein’s general relativity theory.

He recast gravity, not as a force but
as a consequence of the distortion of
space by massive objects such as stars or
galaxies.

Imagine placing a bowling ball
on the middle of a trampoline, and then
rolling a tennis ball past it. The tennis
ball won’t travel in a straight line, but will
instead circle around or fall into the dent
in the centre. This new picture of gravity
explained why a planet orbits a star. It also
predicted that light can’t pass by a massive
object in a straight line. The path of light
through curved space would bend, too.

The first time gravitational lensing
was observed, it rocked the scientific
world. Einstein’s theory predicted that
stars in the same part of the sky as the
sun would appear to be shifted from their
true positions, as the light passing by
the sun would be curved around by its
distortion of space.

During the next total
solar eclipse, when the moon blocked out
the sun’s light, astronomers were able to
see the background stars and measure
the difference, exactly as predicted,
between the stars’ charted positions and
where they appeared. A famous New
York Times headline proclaimed “Lights
All Askew in the Heavens.” Einstein
became an overnight sensation and our
understanding of the nature of space and
time changed forever.

Sometimes that distortion can be so extreme and complex that we see multiple images of the same galaxy, as if looking through warped, uneven glass.

These days, astronomers can use
gravitational lensing to magnify distant
galaxies, helping us see and study the far
reaches of the Universe. In some cases,
it can help us measure the shape of space
itself on the largest scales. It can also map
out invisible dark matter: anything that
has mass distorts space and bends the light
behind it, giving away its presence.

Sometimes that distortion can be so
extreme and complex that we see multiple
images of the same galaxy, as if looking
through warped, uneven glass. Light
originally travelling off to one side might be pulled through a strongly curved part
of space to reach us from another angle. Because a curved path is longer than a
direct one, there can be a delay between
two images of the same distant light
source.

When astronomers discovered
Supernova Refsdal, they knew to be ready
for a replay, because they saw another
image of the host galaxy, but in that
one the supernova had yet to go off. It
involved a fortuitous alignment: the very
distant host galaxy, where the supernova
occurred, and a giant cluster of galaxies
in between. The combined gravity of
an entire cluster turned space into a
distorting lens between the supernova’s
host and us, making the host appear to be
in several places behind it.

After the combined gravity of the
cluster split the image, a single galaxy
acted as another lens, splitting one of the
resulting mirages four more times. When
the original supernova was seen in 2014, it
was seen there, in quadruplicate, framing
the interloper galaxy like a cross. In another
part of the cluster, because the light had
taken a longer route, the doomed star was
still intact. Astronomers calculated the
light travelling on that path would take
about a year more. And then they saw the
explosion, again, right on time.